1. Field of the Invention
The present invention relates to devices for inputting or determining the coordinates (i.e., X and Y coordinates) of a location in a two-dimensional system such as touch sensitive screens for producing output signals related to a touched position. The present invention more particularly provides a resistive touch sensor whereby coordinates of a location can be selected or determined with excellent linearity throughout an increased proportion of the area of the resistive touch sensor. The present invention also more particularly provides a method for producing a touch sensitive screen having reduced bow, or reduced variation of bow, of equipotential field lines therein by selectively modifying the resistance characteristics of a resistive layer on a substrate or gradient sheet.
2. Description of the Prior Art
Touchscreens are becoming the computer input device of choice for an increasing variety of applications. A touchscreen is a transparent input device that can sense the two-dimensional position of the touch of a finger or other electronically passive stylus. Touchscreens are placed over display devices such as cathode-ray-tube monitors and liquid crystal displays. In this manner, touch display systems are provided for many applications including restaurant order entry systems, industrial process control applications, interactive museum exhibits, public information kiosks, lap-top computers used during sales calls, etc.
Many schemes have been proposed for touchscreen construction, some of which have met with commercial success. Presently, the dominant touch technologies are 4-wire resistive 5-wire resistive, capacitive, and ultrasonic. These are technologies that have delivered high standards of performance at cost-competitive prices. Only technologies that can keep pace with downward price pressures will retain a major market share in the future.
5-wire resistive touchscreens, e.g. the AccuTouchTM product line of Elo TouchSystems, Inc. of Fremont, Calif., have been widely accepted for many touchscreen applications. In these touchscreens, mechanical pressure from a finger or stylus causes a plastic membrane coversheet to flex and make physical contact with an underlying glass substrate. The glass substrate is coated with a resistive layer upon which voltage gradients are excited. Via electrical connections to the four comers of the coated glass substrate, associated electronics can sequentially excite gradients in both the X and Y directions. The underside of the coversheet has a conductive coating which provides an electrical connection between the touch location and voltage sensing electronics. Note that there are a total of five electrical connections, i.e., "5 wires", between the touchscreen and the controller electronics. Further details regarding 5-wire resistive touchscreens are found in the following: U.S. Pat. No. 4,220,815 to Gibson; U.S. Pat. Nos. 4,661,655 and 4,731,508 to Gibson et al.; U.S. Pat. No. 4,822,957 to Talmadge et al.; U.S. Pat. No. 5,045,644 to Dunthorn; and U.S. Pat. No. 5,220,136 to Kent, all fully incorporated herein by reference thereto as if repeated verbatim hereinafter.
Manufacturing costs for 4-wire resistive touchscreens are less than for 5-wire resistive touchscreens. 4-wire resistive touchscreens dominate the low-end touch market. However, in applications demanding reliable performance in the face of heavy use, the 5-wire resistive technology has proven to be superior. To measure both X and Y coordinates, 4-wire resistive touchscreens alternate between exciting a voltage gradient on the substrate resistive coating and exciting an orthogonal voltage gradient on the coversheet coating. Performance of 4-wire touchscreens degrade as the uniform resistivity of the coversheet coating is lost as a result of mechanical flexing. This is not a problem for 5-wire touchscreens where both X and Y voltage gradients are generated on the substrate's resistive coating, and the coversheet coating need only provide electrical continuity. However, in a 5-wire touchscreen, a peripheral electrode pattern of some complexity is required to enable sequential generation of both X and Y voltage gradients on the same resistive coating. This is a major reason why the manufacture of 5-wire touchscreens is more costly than the manufacture of 4-wire touchscreens. There is a need to minimize the manufacturing cost of the peripheral electrode pattern.
5-wire resistive touchscreens also compete with non-resistive touch technologies such as ultrasonic touchscreen technology. A performance advantage of the 5-wire touchscreens is their high touch sensitivity for a sharp-tipped passive stylus such as a long fingernail or the comer of a credit card. In contrast, a disadvantage of 5-wire touchscreens is their reduced optical transmission relative to, e.g. an ultrasonic touchscreen. In the design of successful 5-wire resistive touchscreen products, significant attention is given to preserving as much as possible the clarity of the displayed image viewed through the touchscreen. In particular, it is essential that the electrically conductive resistive coatings have high transparency.
A transparent conductive coating is almost an oxymoron. A conductive material is one in which electrons (or holes) are free to move in response to electric fields. A transparent material is one in which electrons are not free to move in response to the electric fields of light radiation. Free electrons reflect light. That is why metals look "metallic." Of great commercial importance is the fact that, due to intricate quantum-mechanical effects, degenerate semi-conductors such as indium tin oxide (ITO) and antimony tin oxide (ATO) provide a means to produce transparent conductive films. We look through degenerate semi-conductors daily when we view liquid crystal displays on digital wristwatches, lap-top computer displays, etc.
Like liquid crystal displays, resistive touchscreens owe their existence as commercially viable products to the quantum mechanics of degenerate semi-conductors. No other material is so well suited to play the role of a transparent resistive coating on a 5-wire touchscreen substrate. 5-wire touchscreen designs of commercial interest must be compatible with the materials and manufacturing processes of degenerate semi-conductors such as ITO.
In the assembly of a 5-wire resistive touchscreen, the most costly component is the substrate, typically about 1.0 mm to about 3.0 mm thick glass, on which has been applied the resistive coating, typically ITO, as well as a peripheral electrode pattern. The peripheral electrode pattern forms a resistor network which is powered at the four corners by excitation voltages from the controller electronics. In turn, the electrode pattern excites voltage gradients in the ITO corresponding to the touchscreen active area. A key to the commercial success of 5-wire resistive touchscreens has been the effort to minimize the cost of this coated and patterned substrate component. Further cost reductions are needed to maintain the competitive position of 5-wire resistive touchscreens.
The substrate design includes conductive traces which connect the four corners of the electrode pattern to a group of soldering pads where a simple five-wire ribbon cable is connected. This reduces the cost of the fully assembled touchscreen by eliminating the need for a complex cable harness and wire routing. A screen-printed silver frit has proven to be the material of choice for these traces due to its high conductivity, durability, and its ability to accept solder connections. Analogous to the plastic insulation of a copper wire, these silver-frit traces are isolated by nearby insulating regions of bare glass substrate. Hence the glass substrate has conductive regions upon which silver frit has been sintered, insulating regions of bare glass, and resistive regions coated with ITO.
Inspection of substrates of commercial products reveals an impressive economy of design in the peripheral electrode pattern. The peripheral electrode pattern is created via clever geometrical arrangements of the three ingredients already present in the substrate design: conductive silver-frit regions, resistive ITO regions, and insulating bare-substrate regions. Furthermore, to control manufacturing costs, the resistive ITO coating in the peripheral electrode region is created in the same manufacturing step and with the same nominal electronic characteristics as in the region in where X and Y voltage gradients are generated. An example is the electrode design given in FIG. 1C herein, reproduced from U.S. Pat. No. 5,045,644 to Dunthorn, fully incorporated herein by reference thereto as if repeated verbatim immediately hereinafter. Such electrode designs that only use the above mentioned materials play a key role in state-of-the-art 5-wire touchscreen technology.
A quality 5-wire touchscreen will generate (X,Y) coordinates that accurately correspond to the position where the finger or stylus activates the touchscreen. This touch-position performance is largely determined by the "linearity" of the touchscreen. In the ITO coating within the touch region of an ideally linear touchscreen, the contours of equal voltage, i.e. equipotential lines, are equally spaced straight lines orthogonal to the X or Y coordinate being measured. Deviations from ideal linearity occur in practice. The design of the peripheral electrode pattern may not be fully optimized. More fundamentally, manufacturing variations in the uniformity of the ITO coating cause deviations from ideal linearity. A central problem for 5-wire resistive technology is to find the most cost-effective way to achieve sufficient linearity to meet marketplace demands for touch position accuracy.
One approach is to insist on tight manufacturing tolerances for the uniformity of the resistivity of the ITO coating. This assures quality product performance but has the disadvantage of driving up the cost of the ITO coating process.
Another approach is to design the peripheral electrode patterns to be more tolerant to variations in ITO resistivity. This approach generally leads to increased current draw through the electrode pattern. This is undesirable in many applications as it places greater power demands on the associated controller electronics. This approach may also lead to an increased width of the peripheral electrode pattern. This is also a major disadvantage for many applications.
Therefore, what is needed and what has been invented is an electrographic touch sensor and method which compensate for batch-to-batch variations in the semiconducting resistive layer and for the limitations of the in-place electrodes. What has been more particularly invented is a resistive touch sensor (i.e., a position touch sensor) and method for controlling the flow of current through a resistive layer for converting physical position information on the resistive layer into electrical signals by modifying the resistance characteristics of the resistive layer.